Some transmission type image display apparatuses, such as liquid crystal display apparatuses employ a direct-type surface light source device as a backlight. In recent years, a direct-type surface light source device including a plurality of light emitting elements as a light source is increasingly used.
A direct-type surface light source device includes, for example, a substrate, a plurality of light emitting elements, a plurality of light flux controlling members (lenses) and a light diffusion member. The light emitting element is a light-emitting diode (LED), such as a white light emitting diode. The plurality of light emitting elements is disposed in a matrix on the substrate. Over each light emitting element, the light flux controlling member is disposed for expanding light emitted from the light emitting element in the surface directions of the substrate. The light emitted from the light flux controlling member is diffused by the light diffusion member, and planarly illuminates a member to be irradiated (e.g. a liquid crystal panel).
FIGS. 1A to 1C illustrate a configuration of a conventional light flux controlling member. FIG. 1A is a perspective view from the rear side, FIG. 1B is a cross-sectional perspective view from the rear side, and FIG. 1C is a cross-sectional view. In FIGS. 1A and 1B, legs formed on the rear side are not illustrated. As illustrated in FIGS. 1A to 1C, conventional light flux controlling member 20 includes incidence surface 22 on which light emitted from a light emitting element is incident and emission surface 24 emitting the light incident on incidence surface 22 toward outside. Incidence surface 22 is a surface with a recessed shape relative to the light emitting element and formed so as to face a light emitting surface of the light emitting element.
FIGS. 2A and 2B are views of optical paths in light flux controlling member 20. FIGS. 2A and 2B illustrate an optical path of a beam emitted from the light emitting center of light emitting element 10 at emission angles of 30° and 40°, respectively. As used herein, “emission angle” is an angle of an emitted beam (θ in FIG. 2A) relative to optical axis OA of light emitting element 10. In FIGS. 2A and 2B, legs formed on the rear side are also not illustrated.
As illustrated in FIGS. 2A and 2B, light emitted from light emitting element 10 enters light flux controlling member 20 from incidence surface 22. The light entering light flux controlling member 20 reaches emission surface 24, and is emitted toward outside from emission surface 24 (solid arrow). At this time, the light is refracted according to the shape of emission surface 24, so that the traveling direction of the light can be controlled. On the other hand, a part of the light reaching emission surface 24 is reflected by emission surface 24 (fresnel reflection) and reaches rear surface 26 facing the substrate on which light emitting element 10 is mounted (dashed arrow). Reflection at rear surface 26 of the light reaching rear surface 26 causes excessive light to travel to immediately above light flux controlling member 20, thereby generating an uneven luminance distribution (uneven luminance) of light emitted from a light emitting device. Emission from rear surface 26 of the light reaching rear surface 26 causes the light to be absorbed into the substrate, thereby increasing the loss of light. PTL 1 proposes a light flux controlling member that can solve such problems.
FIGS. 3A to 3C illustrate a configuration of a light flux controlling member disclosed in PTL 1. FIG. 3A is a perspective view from the rear side, FIG. 3B is a cross-sectional perspective view from the rear side, and FIG. 3C is a cross-sectional view. In FIGS. 3A and 3B, legs formed on the rear side are not illustrated. As illustrated in FIGS. 3A to 3C, in rear surface 26 of light flux controlling member 30 disclosed in PTL 1, formed are a recess including inclining surface 32 on an outer side, and surface 34 substantially parallel to central axis CA on an inner side. Inclining surface 32 is rotationally symmetric (circularly symmetric) about central axis CA of light flux controlling member 30, and inclines at a predetermined angle (e.g. 45°) relative to a virtual straight line orthogonal to central axis CA.
FIGS. 4A and 4B are views of optical paths in light flux controlling member 30. FIGS. 4A and 4B illustrate an optical path of a beam emitted from the light emitting center of light emitting element 10 at emission angles of 30° and 40°, respectively. In FIGS. 4A and 4B, legs formed on the rear side are also not illustrated. As illustrated in FIGS. 4A and 4B, fresnel-reflected light at emission surface 24 reaches a predetermined region on rear surface 26. Forming inclining surface 32 in the predetermined region enables changing the direction of at least a part of the light reaching inclining surface 32 by reflection to a lateral direction (see FIGS. 4A and 4B).
In light flux controlling member 30 disclosed in PTL 1, the light reflected by emission surface 24 is less likely to become light traveling to immediately above light flux controlling member 30, or is less likely to be absorbed into the substrate. Therefore, a light emitting device including light flux controlling member 30 disclosed in PTL 1 can radiate light uniformly and efficiently compared to a light emitting device including a conventional light flux controlling member.
In recent years, chip-on-board (COB) LEDs are used for lighting due to their easy mounting and high light emission efficiency. The COB LEDs are known to emit more light in lateral directions than conventional LEDs do, in addition to emission of light upward.